1. Field of the Invention
The present invention relates to a stainless steel vacuum double-walled container for use as a portable thermos bottle, pot, jar and the like.
2. Relevant Art
Vacuum double-walled containers manufactured from stainless steel require a sufficient mechanical strength to prevent exterior damage or deformation due to impacts sustained when dropped or acted on by an exterior force: in particular, containers for use as portable thermos bottles require an increased strength. The strength of the container is approximately proportional to the hardness of the stainless steel plate comprising the container. Consequently, by using a stainless steel with a sufficiently high strength, the thickness of the stainless steel plate material comprising the double-walled container can be reduced, and as a result it is possible to significantly decrease the overall weight of the stainless steel vacuum double-walled container.
With regard to increasing the strength of the stainless steel plate, a method exists in which the draft of the stainless steel component is increased, in order to increase the curing operation (work hardening), by performing refining and/or martensite transformation of the metallic crystal granules during the metal processing.
On the other hand, as a process for vacuum sealing a stainless steel vacuum double-walled container, vacuum sealing processes are known such as those stated in Japanese patent application, first publication No. Sho 59-103633, Japanese patent application, first publication No. Sho 61-106119, or Japanese patent application, first publication No. Sho 58-192516 in which pressure welding and sealing of a chip tube is performed after creating a predetermined degree of vacuum by discharging the air via a chip tube arranged in the outer container at a temperature of 400.degree. C..about.1200.degree. C. under vacuum or atmospheric air conditions, or in which vacuum sealing is carried out by heat processing and then melting a solid brazing material.
However, according to a conventional method for manufacturing a stainless steel vacuum double-walled container, a container is formed by processing a cold-rolled steel plate, e.g. a stainless steel plate with a surface finish such as a No. 2B material according to the JIS standard, by means of rolling, drawing, welding or a similar process. The hardness therein is determined by work processing in this manner, and consequently it is not possible to reduce the plate thickness below the minimum plate thickness required for each respective process.
Furthermore, there are also times when the stainless steel of double-walled containers, which are work hardened by means of heat processing during vacuum sealing, reduce the hardness as a result of annealing. In other words, stainless steel imparted with a high hardness from work hardening is softened when annealed at a temperature of 600.degree. C. or greater. In addition, at a temperature of 600.degree..about.800.degree. C., the solid solution carbon precipitates out as a carbide resulting in sensitization, easily occurrence of intergranular corrosion, and reduction of the corrosion resistance. Furthermore, the hardness is further reduced when the carbide is transformed into a solid solution by heating at a temperature of 1000.degree..about.1200.degree. C. and returned to an austenite structure. The application of heat during vacuum discharge is important in order to sufficiently discharge the residue gas component in the stainless steel and create a high vacuum. This application of heat requires a temperature at least that of practical use: e.g. in the case of a thermos bottle, a temperature of 100.degree. C. or greater is required in order to store the boiling water. From the aforementioned, in the manufacturing of a light-weight, vacuum double-walled container it is necessary to increase the hardness of the stainless steel material to be used, take into consideration the corrosion resistance according to heat processing, and select conditions which will prevent softening of the stainless steel.